CHAPTER I. 

 SENSORY NERVES. 



IN studying the phenomena of motor nerves we are greatly assisted 

 by two facts : First, that the muscular contraction by which we 

 judge of what is going on in the nerve is a comparatively simple 

 thing, one contraction differing from another only by such features 

 as amount, rapidity, and frequency of repetition, and all such 

 differences being capable of exact measurement. Secondly, that 

 when we apply a stimulus directly to the nerve itself, the effects 

 differ in degree only from those which result when the nerve is set 

 in action by natural stimuli, such as the will. When we come, on 

 the other hand, to investigate the phenomena of afferent nerves, 

 our labours are for the time rendered heavier, but in the end 

 more fruitful, by the facts : First, that we can only judge of 

 what is going on in an afferent nerve by the effects it produces 

 in some central nervous organ, in the way of exciting or modifying 

 reflex action, or modifying automatic action, or affecting conscious- 

 ness ; and we are consequently met on the very threshold of every 

 inquiry by the difficulty of clearly distinguishing the events which 

 belong exclusively to the afferent nerve from those which belong to 

 the central organ. Secondly, that the effects of applying a stimulus 

 to the peripheral end-organ of an afferent nerve are very different 

 from those of applying the same stimulus directly to the nerve- 

 trunk. This may be shewn by the simple experience of comparing 

 the sensation caused by the contact of any sharp body with a 

 nerve laid bare by a wound with that caused by contact of an intact 

 skin with the same body. These differences reveal to us a com- 

 plexity of impulses, of which the phenomena of motor nerves gave 

 us not so much as a hint ; but for the time being they increase the 

 difficulties of our study. 



F. 31 



